Heat spreader for plasma display panel

ABSTRACT

The invention presented is a method for applying heat spreaders to a plurality of plasma display panels, including (a) providing a plurality of heat spreader composites, each of which comprises a heat spreader material having an adhesive thereon and a release material positioned such that the adhesive is sandwiched between the heat spreader material and the release material; (b) removing the release material from a plurality of the composites; and (c) applying at least one of the composites to each of the plurality of plasma display panels each such that the adhesive adheres the heat spreader material to the plasma display panel.

RELATED APPLICATION

This application is a continuation under 35 U.S.C. §120 from U.S. patentapplication Ser. No. 10/685,103 filed Oct. 14, 2003, entitled “HeatSpreader for Plasma Display Panel,” now U.S. Pat. No. 7,138,029 andwhich is hereby incorporated by reference.

TECHNICAL FIELD

The present invention relates to a heat spreader useful for a plasmadisplay panel for and a method of applying the inventive heat spreaderto a plasma display panel.

BACKGROUND OF THE ART

A plasma display panel is a display apparatus which contains a pluralityof discharge cells, and is constructed to display an image by applying avoltage across electrodes discharge cells thereby causing the desireddischarge cell to emit light. A panel unit, which is the main part ofthe plasma display panel, is fabricated by bonding two glass base platestogether in such a manner as to sandwich a plurality of discharge cellsbetween them.

In a plasma display panel, the discharge cells which are caused to emitlight for image formation generate heat and each thus constitutes asource of heat, which causes the temperature of the plasma display panelas a whole to rise. The heat generated in the discharge cells istransferred to the glass forming the base plates, but heat conduction indirections parallel to the panel face is difficult because of theproperties of the glass base plate material.

In addition, the temperature of a discharge cell which has beenactivated for light emission rises markedly, while the temperature of adischarge cell which has not been activated does not rise as much.Because of this, the panel face temperature of the plasma display panelrises locally in the areas where an image is being generated,accelerating thermal deterioration of affected discharge cells, unlesssome heat sinking measures are taken.

Further, since the temperature difference between activated andnonactivated discharge cells can be high, and, in fact, the temperaturedifference between discharge cells generating white light and thosegenerating darker colors also can be high, a stress is applied to thepanel unit, causing the conventional plasma display panel to be prone tocracks and breakage.

When the voltage to be applied to the electrodes of discharge cells isincreased, the brightness of the discharge cells increases but theamount of heat generation in such cells also increases. Thus, thosecells having large voltages for activation become more susceptible tothermal deterioration and tend to exacerbate the breakage problem of thepanel unit of the plasma display panel.

The use of graphite films or sheets as thermal interface materials forplasma display panels has been suggested by, for example, Morita,Ichiyanagi, Ikeda, Nishiki, Inoue, Komyoji and Kawashima in U.S. Pat.No. 5,831,374. In addition, the heat spreading capabilities of sheets ofcompressed particles of exfoliated graphite has also been recognized.Indeed, such materials are commercially available from Advanced EnergyTechnology Inc. of Lakewood, Ohio as its eGraf® 700 class of materials.

Graphites are made up of layer planes of hexagonal arrays or networks ofcarbon atoms. These layer planes of hexagonally arranged carbon atomsare substantially flat and are oriented or ordered so as to besubstantially parallel and equidistant to one another. The substantiallyflat, parallel equidistant sheets or layers of carbon atoms, usuallyreferred to as graphene layers or basal planes, are linked or bondedtogether and groups thereof are arranged in crystallites. Highly orderedgraphites consist of crystallites of considerable size, the crystallitesbeing highly aligned or oriented with respect to each other and havingwell ordered carbon layers. In other words, highly ordered graphiteshave a high degree of preferred crystallite orientation. It should benoted that graphites possess anisotropic structures and thus exhibit orpossess many properties that are highly directional such as thermal andelectrical conductivity.

Briefly, graphites may be characterized as laminated structures ofcarbon, that is, structures consisting of superposed layers or laminaeof carbon atoms joined together by weak van der Waals forces. Inconsidering the graphite structure, two axes or directions are usuallynoted, to wit, the “c” axis or direction and the “a” axes or directions.For simplicity, the “c” axis or direction may be considered as thedirection perpendicular to the carbon layers. The “a” axes or directionsmay be considered as the directions parallel to the carbon layers or thedirections perpendicular to the “c” direction. The graphites suitablefor manufacturing flexible graphite sheets possess a very high degree oforientation.

As noted above, the bonding forces holding the parallel layers of carbonatoms together are only weak van der Waals forces. Natural graphites canbe treated so that the spacing between the superposed carbon layers orlaminae can be appreciably opened up so as to provide a marked expansionin the direction perpendicular to the layers, that is, in the “c”direction, and thus form an expanded or intumesced graphite structure inwhich the laminar character of the carbon layers is substantiallyretained.

Graphite flake which has been greatly expanded and more particularlyexpanded so as to have a final thickness or “c” direction dimensionwhich is as much as about 80 or more times the original “c” directiondimension can be formed without the use of a binder into cohesive orintegrated sheets of expanded graphite, e.g. webs, papers, strips,tapes, foils, mats or the like (typically referred to as “flexiblegraphite”). The formation of graphite particles which have been expandedto have a final thickness or “c” dimension which is as much as about 80times or more the original “c” direction dimension into integratedflexible sheets by compression, without the use of any binding material,is believed to be possible due to the mechanical interlocking, orcohesion, which is achieved between the voluminously expanded graphiteparticles.

In addition to flexibility, the sheet material, as noted above, has alsobeen found to possess a high degree of anisotropy with respect tothermal conductivity due to orientation of the expanded graphiteparticles and graphite layers substantially parallel to the opposedfaces of the sheet resulting from high compression, making it especiallyuseful in heat spreading applications. Sheet material thus produced hasexcellent flexibility, good strength and a high degree of orientation.

Briefly, the process of producing flexible, binderless anisotropicgraphite sheet material, e.g. web, paper, strip, tape, foil, mat, or thelike, comprises compressing or compacting under a predetermined load andin the absence of a binder, expanded graphite particles which have a “c”direction dimension which is as much as about 80 or more times that ofthe original particles so as to form a substantially flat, flexible,integrated graphite sheet. The expanded graphite particles thatgenerally are worm-like or vermiform in appearance, once compressed,will maintain the compression set and alignment with the opposed majorsurfaces of the sheet. The density and thickness of the sheet materialcan be varied by controlling the degree of compression. The density ofthe sheet material can be within the range of from about 0.04 g/cc toabout 2.0 g/cc.

The flexible graphite sheet material exhibits an appreciable degree ofanisotropy due to the alignment of graphite particles parallel to themajor opposed, parallel surfaces of the sheet, with the degree ofanisotropy increasing upon compression of the sheet material to increaseorientation. In compressed anisotropic sheet material, the thickness,i.e. the direction perpendicular to the opposed, parallel sheet surfacescomprises the “c” direction and the directions ranging along the lengthand width, i.e. along or parallel to the opposed, major surfacescomprises the “a” directions and the thermal and electrical propertiesof the sheet are very different, by orders of magnitude, for the “c” and“a” directions.

One drawback to the use of graphite sheets as heat spreaders for plasmadisplay panels lies in the plasma display panel manufacturing process.More specifically, plasma display panels are produced in very highvolumes, and the process for applying a graphite heat spreader to theplasma display panel need be such that a bottleneck in the manufacturingprocess is not created. Moreover, a means of adhering the graphitespreader to the panel is needed to avoid having the graphite spreaderfall off during the manufacturing process and to ensure good thermalcontact between the graphite spreader and the plasma display panelwithout the requirement of high pressure application of the spreader;however, the attachment method must not have a significant deleteriousimpact on the thermal performance of the heat spreader.

One method for attaching a graphite heat spreader to a plasma displaypanel is by use of an adhesive applied to the graphite. U.S. Pat. No.6,245,400 to Tzeng, Getz and Weber describes a method for producing arelease-lined pressure sensitive adhesive flexible graphite sheetarticle, wherein the release liner is easily removed from the graphitesheet without delaminating the graphite. Graphite sheet has a relativelylow cohesive strength and removing the release liner withoutdelaminating the graphite is a significant challenge. A key component ofthe Tzeng et al. patent is the use of a primer coating applied to thegraphite sheet prior to applying the pressure sensitive adhesive. Thedisadvantage of this approach is the need for an additional coatingstep, which increases manufacturing complexity and cost.

Thus, a method for producing a release lined pressure sensitive adhesiveflexible graphite sheet without the use of a primer coating is neededfor use as a heat spreader for plasma display panels. Furthermore, thismethod should allow for the achievement of very high speed of release ofthe release liner from the adhesive-coated graphite sheet, withoutdelamination of the graphite, and yet not create an undesirably highreduction of the thermal properties of the graphite heat spreader.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide amethod for applying heat spreaders to plasma display panels in a highvolume manufacturing process.

Another object of the present invention is to provide a heat spreadermaterial which can be used in a high volume plasma display panelmanufacturing process.

Yet another object of the present invention is to provide a method forapplying a heat spreader material to a heat source such as a plasmadisplay panel in a high volume manufacturing process where theapplication of the heat spreader does not create a bottleneck in themanufacturing process.

Another object of the present invention is to provide a heat spreadermaterial which can be applied to a heat source or collection of heatsources such as a plasma display panel and adhere with good thermalcontact between the heat spreader and the plasma display panel withoutthe requirement of high pressure application of the spreader to achievethe desired thermal contact.

Still another object of the present invention is to provide a heatspreader material which can be applied to a heat source or collection ofheat sources such as a plasma display panel and adhere to the sourcewithout falling off during the assembly process.

A further object of the present invention is to provide a method forapplying a heat spreader to a heat source or collection of heat sourcessuch as a plasma display panel which does not significantly impact thethermal performance of the heat spreader.

These objects and others which will be apparent to the skilled artisanupon reading the following description, can be achieved by providing amethod for applying heat spreaders to a heat source, such as a pluralityof plasma display panels, where the method includes providing aplurality of heat spreader composites, each of which comprises a heatspreader material having an adhesive thereon and a release materialpositioned such that the adhesive is sandwiched between the heatspreader material and the release material; removing the releasematerial from a plurality of the composites; and applying at least oneof the composites to each of the plurality of plasma display panels suchthat the adhesive adheres the heat spreader material to the plasmadisplay panel.

The release material and adhesive should each be selected to permit apredetermined rate of release of the release material without causingundesirable damage to the heat spreader material. In addition, theadhesive and release material should also provide an average releaseload of no greater than about 40 grams per centimeter (g/cm) at arelease speed of about 1 meter/second (m/s). Indeed, the average releaseload should be no greater than about 20 g/cm, most preferably no greaterthan about 10 g/cm, at a release speed of about 1 m/s.

Furthermore, the adhesive should preferably achieve a minimum lap shearadhesion strength of at least about 125 g/cm², more preferably anaverage lap shear adhesion strength of at least about 700 g/cm².

In order to avoid unduly high thermal losses, the adhesive should resultin an increase in through-thickness thermal resistance of theadhesive/heat spreader material combination of not more than about 100%and preferably not more than about 35% as compared to the heat spreadermaterial itself. To meet the requirements of liner release speed,adhesion strength and thermal resistance, the adhesive thickness shouldbe no greater than about 0.5 mils in thickness and most preferablybetween about 0.1 and about 0.25 mils in thickness.

The heat spreader material advantageously comprises graphite, especiallyat least one sheet of compressed particles of expanded graphite, whichcan be provided as a laminate comprising a plurality of sheets ofcompressed particles of expanded graphite.

Other and further objects, features and advantages of the presentinvention will be readily apparent to those skilled in the art upon areading of the following disclosure when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of the high speed release test of Example 1.

FIG. 2 is a diagram of the high speed release test of Example 1, shownduring testing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Graphite is a crystalline form of carbon comprising atoms covalentlybonded in flat layered planes with weaker bonds between the planes. Inobtaining source materials such as the above flexible sheets ofgraphite, particles of graphite, such as natural graphite flake, aretypically treated with an intercalant of, e.g. a solution of sulfuricand nitric acid, where the crystal structure of the graphite reacts toform a compound of graphite and the intercalant. The treated particlesof graphite are hereafter referred to as “particles of intercalatedgraphite.” Upon exposure to high temperature, the intercalant within thegraphite decomposes and volatilizes, causing the particles ofintercalated graphite to expand in dimension as much as about 80 or moretimes its original volume in an accordion-like fashion in the “c”direction, i.e. in the direction perpendicular to the crystalline planesof the graphite. The expanded (otherwise referred to as exfoliated)graphite particles are vermiform in appearance, and are thereforecommonly referred to as worms. The worms may be compressed together intoflexible sheets that, unlike the original graphite flakes, can be formedand cut into various shapes and provided with small transverse openingsby deforming mechanical impact.

Graphite starting materials for the flexible sheets suitable for use inthe present invention include highly graphitic carbonaceous materialscapable of intercalating organic and inorganic acids as well as halogensand then expanding when exposed to heat. These highly graphiticcarbonaceous materials most preferably have a degree of graphitizationof about 1.0. As used in this disclosure, the term “degree ofgraphitization” refers to the value g according to the formula:

$g = \frac{3.45 - {d(002)}}{0.095}$where d(002) is the spacing between the graphitic layers of the carbonsin the crystal structure measured in Angstrom units. The spacing dbetween graphite layers is measured by standard X-ray diffractiontechniques. The positions of diffraction peaks corresponding to the(002), (004) and (006) Miller Indices are measured, and standardleast-squares techniques are employed to derive spacing which minimizesthe total error for all of these peaks. Examples of highly graphiticcarbonaceous materials include natural graphites from various sources,as well as other carbonaceous materials such as graphite prepared bychemical vapor deposition, high temperature pyrolysis of polymers, orcrystallization from molten metal solutions, and the like. Naturalgraphite is most preferred.

The graphite starting materials for the flexible sheets used in thepresent invention may contain non-graphite components so long as thecrystal structure of the starting materials maintains the requireddegree of graphitization and they are capable of exfoliation. Generally,any carbon-containing material, the crystal structure of which possessesthe required degree of graphitization and which can be exfoliated, issuitable for use with the present invention. Such graphite preferablyhas an ash content of less than twenty weight percent. More preferably,the graphite employed for the present invention will have a purity of atleast about 94%. In the most preferred embodiment, the graphite employedwill have a purity of at least about 98%.

A common method for manufacturing graphite sheet is described by Shaneet al. in U.S. Pat. No. 3,404,061, the disclosure of which isincorporated herein by reference. In the typical practice of the Shaneet al. method, natural graphite flakes are intercalated by dispersingthe flakes in a solution containing e.g., a mixture of nitric andsulfuric acid, advantageously at a level of about 20 to about 300 partsby weight of intercalant solution per 100 parts by weight of graphiteflakes (pph). The intercalation solution contains oxidizing and otherintercalating agents known in the art. Examples include those containingoxidizing agents and oxidizing mixtures, such as solutions containingnitric acid, potassium chlorate, chromic acid, potassium permanganate,potassium chromate, potassium dichromate, perchloric acid, and the like,or mixtures, such as for example, concentrated nitric acid and chlorate,chromic acid and phosphoric acid, sulfuric acid and nitric acid, ormixtures of a strong organic acid, e.g. trifluoroacetic acid, and astrong oxidizing agent soluble in the organic acid. Alternatively, anelectric potential can be used to bring about oxidation of the graphite.Chemical species that can be introduced into the graphite crystal usingelectrolytic oxidation include sulfuric acid as well as other acids.

In a preferred embodiment, the intercalating agent is a solution of amixture of sulfuric acid, or sulfuric acid and phosphoric acid, and anoxidizing agent, i.e. nitric acid, perchloric acid, chromic acid,potassium permanganate, hydrogen peroxide, iodic or periodic acids, orthe like. Although less preferred, the intercalation solution maycontain metal halides such as ferric chloride, and ferric chloride mixedwith sulfuric acid, or a halide, such as bromine as a solution ofbromine and sulfuric acid or bromine in an organic solvent.

The quantity of intercalation solution may range from about 20 to about350 pph and more typically about 40 to about 160 pph. After the flakesare intercalated, any excess solution is drained from the flakes and theflakes are water-washed.

Alternatively, the quantity of the intercalation solution may be limitedto between about 10 and about 40 pph, which permits the washing step tobe eliminated as taught and described in U.S. Pat. No. 4,895,713, thedisclosure of which is also herein incorporated by reference.

The particles of graphite flake treated with intercalation solution canoptionally be contacted, e.g. by blending, with a reducing organic agentselected from alcohols, sugars, aldehydes and esters which are reactivewith the surface film of oxidizing intercalating solution attemperatures in the range of 25° C. and 125° C. Suitable specificorganic agents include hexadecanol, octadecanol, 1-octanol, 2-octanol,decylalcohol, 1,10decanediol, decylaldehyde, 1-propanol, 1,3propanediol,ethyleneglycol, polypropylene glycol, dextrose, fructose, lactose,sucrose, potato starch, ethylene glycol monostearate, diethylene glycoldibenzoate, propylene glycol monostearate, glycerol monostearate,dimethyl oxylate, diethyl oxylate, methyl formate, ethyl formate,ascorbic acid and lignin-derived compounds, such as sodium lignosulfate.The amount of organic reducing agent is suitably from about 0.5 to 4% byweight of the particles of graphite flake.

The use of an expansion aid applied prior to, during or immediatelyafter intercalation can also provide improvements. Among theseimprovements can be reduced exfoliation temperature and increasedexpanded volume (also referred to as “worm volume”). An expansion aid inthis context will advantageously be an organic material sufficientlysoluble in the intercalation solution to achieve an improvement inexpansion. More narrowly, organic materials of this type that containcarbon, hydrogen and oxygen, preferably exclusively, may be employed.Carboxylic acids have been found especially effective. A suitablecarboxylic acid useful as the expansion aid can be selected fromaromatic, aliphatic or cycloaliphatic, straight chain or branched chain,saturated and unsaturated monocarboxylic acids, dicarboxylic acids andpolycarboxylic acids which have at least 1 carbon atom, and preferablyup to about 15 carbon atoms, which is soluble in the intercalationsolution in amounts effective to provide a measurable improvement of oneor more aspects of exfoliation. Suitable organic solvents can beemployed to improve solubility of an organic expansion aid in theintercalation solution.

Representative examples of saturated aliphatic carboxylic acids areacids such as those of the formula H(CH₂)_(n)COOH wherein n is a numberof from 0 to about 5, including formic, acetic, propionic, butyric,pentanoic, hexanoic, and the like. In place of the carboxylic acids, theanhydrides or reactive carboxylic acid derivatives such as alkyl esterscan also be employed. Representative of alkyl esters are methyl formateand ethyl formate. Sulfuric acid, nitric acid and other known aqueousintercalants have the ability to decompose formic acid, ultimately towater and carbon dioxide. Because of this, formic acid and othersensitive expansion aids are advantageously contacted with the graphiteflake prior to immersion of the flake in aqueous intercalant.Representative of dicarboxylic acids are aliphatic dicarboxylic acidshaving 2-12 carbon atoms, in particular oxalic acid, fumaric acid,malonic acid, maleic acid, succinic acid, glutaric acid, adipic acid,1,5-pentanedicarboxylic acid, 1,6-hexanedicarboxylic acid,1,10-decanedicarboxylic acid, cyclohexane-1,4-dicarboxylic acid andaromatic dicarboxylic acids such as phthalic acid or terephthalic acid.Representative of alkyl esters are dimethyl oxylate and diethyl oxylate.Representative of cycloaliphatic acids is cyclohexane carboxylic acidand of aromatic carboxylic acids are benzoic acid, naphthoic acid,anthranilic acid, p-aminobenzoic acid, salicylic acid, o-, m- andp-tolyl acids, methoxy and ethoxybenzoic acids, acetoacetamidobenzoicacids and, acetamidobenzoic acids, phenylacetic acid and naphthoicacids. Representative of hydroxy aromatic acids are hydroxybenzoic acid,3-hydroxy-1-naphthoic acid, 3-hydroxy-2-naphthoic acid,4-hydroxy-2-naphthoic acid, 5-hydroxy-1-naphthoic acid,5-hydroxy-2-naphthoic acid, 6-hydroxy-2-naphthoic acid and7-hydroxy-2-naphthoic acid. Prominent among the polycarboxylic acids iscitric acid.

The intercalation solution will be aqueous and will preferably containan amount of expansion aid of from about 1 to 10%, the amount beingeffective to enhance exfoliation. In the embodiment wherein theexpansion aid is contacted with the graphite flake prior to or afterimmersing in the aqueous intercalation solution, the expansion aid canbe admixed with the graphite by suitable means, such as a V-blender,typically in an amount of from about 0.2% to about 10% by weight of thegraphite flake.

After intercalating the graphite flake, and following the blending ofthe intercalated graphite flake with the organic reducing agent, theblend can be exposed to temperatures in the range of 25° to 125° C. topromote reaction of the reducing agent and intercalated graphite flake.The heating period is up to about 20 hours, with shorter heatingperiods, e.g., at least about 10 minutes, for higher temperatures in theabove-noted range. Times of one-half hour or less, e.g., on the order of10 to 25 minutes, can be employed at the higher temperatures.

The above described methods for intercalating and exfoliating graphiteflake may beneficially be augmented by a pretreatment of the graphiteflake at graphitization temperatures, i.e. temperatures in the range ofabout 3000° C. and above and by the inclusion in the intercalant of alubricious additive.

The pretreatment, or annealing, of the graphite flake results insignificantly increased expansion (i.e., increase in expansion volume ofup to 300% or greater) when the flake is subsequently subjected tointercalation and exfoliation. Indeed, desirably, the increase inexpansion is at least about 50%, as compared to similar processingwithout the annealing step. The temperatures employed for the annealingstep should not be significantly below 3000° C., because temperatureseven 100° C. lower result in substantially reduced expansion.

The annealing of the present invention is performed for a period of timesufficient to result in a flake having an enhanced degree of expansionupon intercalation and subsequent exfoliation. Typically the timerequired will be 1 hour or more, preferably 1 to 3 hours and will mostadvantageously proceed in an inert environment. For maximum beneficialresults, the annealed graphite flake will also be subjected to otherprocesses known in the art to enhance the degree expansion—namelyintercalation in the presence of an organic reducing agent, anintercalation aid such as an organic acid, and a surfactant washfollowing intercalation. Moreover, for maximum beneficial results, theintercalation step may be repeated.

The annealing step of the instant invention may be performed in aninduction furnace or other such apparatus as is known and appreciated inthe art of graphitization; for the temperatures here employed, which arein the range of 3000° C., are at the high end of the range encounteredin graphitization processes.

Because it has been observed that the worms produced using graphitesubjected to pre-intercalation annealing can sometimes “clump” together,which can negatively impact area weight uniformity, an additive thatassists in the formation of “free flowing” worms is highly desirable.The addition of a lubricious additive to the intercalation solutionfacilitates the more uniform distribution of the worms across the bed ofa compression apparatus (such as the bed of a calender stationconventionally used for compressing (or “calendering”) graphite wormsinto flexible graphite sheet. The resulting sheet therefore has higherarea weight uniformity and greater tensile strength, even when thestarting graphite particles are smaller than conventionally used. Thelubricious additive is preferably a long chain hydrocarbon. Otherorganic compounds having long chain hydrocarbon groups, even if otherfunctional groups are present, can also be employed.

More preferably, the lubricious additive is an oil, with a mineral oilbeing most preferred, especially considering the fact that mineral oilsare less prone to rancidity and odors, which can be an importantconsideration for long term storage. It will be noted that certain ofthe expansion aids detailed above also meet the definition of alubricious additive. When these materials are used as the expansion aid,it may not be necessary to include a separate lubricious additive in theintercalant.

The lubricious additive is present in the intercalant in an amount of atleast about 1.4 pph, more preferably at least about 1.8 pph. Althoughthe upper limit of the inclusion of lubricous additive is not ascritical as the lower limit, there does not appear to be any significantadditional advantage to including the lubricious additive at a level ofgreater than about 4 pph.

The thus treated particles of graphite are sometimes referred to as“particles of intercalated graphite.” Upon exposure to high temperature,e.g. temperatures of at least about 160° C. and especially about 700° C.to 1000° C. and higher, the particles of intercalated graphite expand asmuch as about 80 to 1000 or more times their original volume in anaccordion-like fashion in the c-direction, i.e. in the directionperpendicular to the crystalline planes of the constituent graphiteparticles. The expanded, i.e. exfoliated, graphite particles arevermiform in appearance, and are therefore commonly referred to asworms. The worms may be compression molded together into flexible sheetshaving small transverse openings that, unlike the original graphiteflakes, can be formed and cut into various shapes, as hereinafterdescribed.

Alternatively, the flexible graphite sheets of the present invention mayutilize particles of reground flexible graphite sheets rather thanfreshly expanded worms. The sheets may be newly formed sheet material,recycled sheet material, scrap sheet material, or any other suitablesource.

Also the processes of the present invention may use a blend of virginmaterials and recycled materials.

The source material for recycled materials may be sheets or trimmedportions of sheets that have been compression molded as described above,or sheets that have been compressed with, for example, pre-calenderingrolls, but have not yet been impregnated with resin. Furthermore, thesource material may be sheets or trimmed portions of sheets that havebeen impregnated with resin, but not yet cured, or sheets or trimmedportions of sheets that have been impregnated with resin and cured. Thesource material may also be recycled flexible graphite PEM fuel cellcomponents such as flow field plates or electrodes. Each of the varioussources of graphite may be used as is or blended with natural graphiteflakes.

Once the source material of flexible graphite sheets is available, itcan then be comminuted by known processes or devices, such as a jetmill, air mill, blender, etc. to produce particles. Preferably, amajority of the particles have a diameter such that they will passthrough 20 U.S. mesh; more preferably a major portion (greater thanabout 20%, most preferably greater than about 50%) will not pass through80 U.S. mesh. Most preferably the particles have a particle size of nogreater than about 20 mesh. It may be desirable to cool the flexiblegraphite sheet when it is resin-impregnated as it is being comminuted toavoid heat damage to the resin system during the comminution process.

The size of the comminuted particles may be chosen so as to balancemachinability and formability of the graphite article with the thermalcharacteristics desired. Thus, smaller particles will result in agraphite article which is easier to machine and/or form, whereas largerparticles will result in a graphite article having higher anisotropy,and, therefore, greater in-plane electrical and thermal conductivity.

If the source material has been resin impregnated, then preferably theresin is removed from the particles. Details of the resin removal arefurther described below.

Once the source material is comminuted, and any resin is removed, it isthen re-expanded. The re-expansion may occur by using the intercalationand exfoliation process described above and those described in U.S. Pat.No. 3,404,061 to Shane et al. and U.S. Pat. No. 4,895,713 to Greinke etal.

Typically, after intercalation the particles are exfoliated by heatingthe intercalated particles in a furnace. During this exfoliation step,intercalated natural graphite flakes may be added to the recycledintercalated particles. Preferably, during the re-expansion step theparticles are expanded to have a specific volume in the range of atleast about 100 cc/g and up to about 350 cc/g or greater. Finally, afterthe re-expansion step, the re-expanded particles may be compressed intoflexible sheets, as hereinafter described.

If the starting material has been impregnated with a resin, the resinshould preferably be at least partially removed from the particles. Thisremoval step should occur between the comminuting step and there-expanding step.

In one embodiment, the removing step includes heating the resincontaining regrind particles, such as over an open flame. Morespecifically, the impregnated resin may be heated to a temperature of atleast about 250° C. to effect resin removal. During this heating stepcare should be taken to avoid flashing of the resin decompositionproducts; this can be done by careful heating in air or by heating in aninert atmosphere. Preferably, the heating should be in the range of fromabout 400° C. to about 800° C. for a time in the range of from at leastabout 10 and up to about 150 minutes or longer.

Additionally, the resin removal step may result in increased tensilestrength of the resulting article produced from the molding process ascompared to a similar method in which the resin is not removed. Theresin removal step may also be advantageous because during the expansionstep (i.e., intercalation and exfoliation), when the resin is mixed withthe intercalation chemicals, it may in certain instances create toxicbyproducts.

Thus, by removing the resin before the expansion step a superior productis obtained such as the increased strength characteristics discussedabove. The increased strength characteristics are a result of in partbecause of increased expansion. With the resin present in the particles,expansion may be restricted.

In addition to strength characteristics and environmental concerns,resin may be removed prior to intercalation in view of concerns aboutthe resin possibly creating a run away exothermic reaction with theacid.

In view of the above, preferably a majority of the resin is removed.More preferably, greater than about 75% of the resin is removed. Mostpreferably, greater than 99% of the resin is removed.

Once the flexible graphite sheet is comminuted, it is formed into thedesired shape (i.e., a sheet) and then cured (when resin impregnated) inthe preferred embodiment. Alternatively, the sheet can be cured prior tobeing comminuted, although post-comminution cure is preferred.

Flexible graphite sheet and foil are coherent, with good handlingstrength, and are suitably compressed by, e.g. compression molding, to athickness of about 0.025 mm to 3.75 mm and a typical density of about0.1 to 1.5 grams per cubic centimeter (g/cc). From about 1.5-30% byweight of ceramic additives can be blended with the intercalatedgraphite flakes as described in U.S. Pat. No. 5,902,762 (which isincorporated herein by reference) to provide enhanced resin impregnationin the final flexible graphite product. The additives include ceramicfiber particles having a length of about 0.15 to 1.5 millimeters. Thewidth of the particles is suitably from about 0.04 to 0.004 mm. Theceramic fiber particles are non-reactive and non-adhering to graphiteand are stable at temperatures up to about 1100° C., preferably about1400° C. or higher. Suitable ceramic fiber particles are formed ofmacerated quartz glass fibers, carbon and graphite fibers, zirconia,boron nitride, silicon carbide and magnesia fibers, naturally occurringmineral fibers such as calcium metasilicate fibers, calcium aluminumsilicate fibers, aluminum oxide fibers and the like.

The flexible graphite sheet can also, at times, be advantageouslytreated with resin and the absorbed resin, after curing, enhances themoisture resistance and handling strength, i.e. stiffness, of theflexible graphite sheet as well as “fixing” the morphology of the sheet.Suitable resin content is preferably at least about 5% by weight, morepreferably about 10 to 35% by weight, and suitably up to about 60% byweight. Resins found especially useful in the practice of the presentinvention include acrylic-, epoxy- and phenolic-based resin systems, ormixtures thereof. Suitable epoxy resin systems include those based ondiglycidyl ether or bisphenol A (DGEBA) and other multifunctional resinsystems; phenolic resins that can be employed include resole and novolakphenolics.

Although this application is written in terms of the application of heatspreaders to plasma display panels, it will be recognized that theinventive method and heat sp reader are equally applicable to other heatsources, or heat source collections, especially those being manufacturedin high volume processes.

Plasma display panels are now being produced at sizes of 1 meter andabove (measured from corner to corner). Thus, heat spreaders used tocool and ameliorate the effects of hot spots on such panels are alsorequired to be relatively large, on the order of about 270millimeters×about 500 millimeters, or as large as about 800millimeters×500 millimeters, or even larger. In a plasma display panel,as discussed above, hundreds of thousands of cells, each containing aplasma gas, are present. When a voltage is applied to each cell, theplasma gas then reacts with phosphors in each cell to produce coloredlight. Since significant power is required to ionize the gas to producethe plasma, the plasma display can become very hot. Moreover, dependingon the color in a particular region of the panel, hot spots can becreated on the screen which can result in premature breakdown of thephosphors which can shorten display life as well as cause thermalstresses on the panel itself. Therefore, a heat spreader is needed toreduce the effect of these hot spots.

Sheets of compressed particles of exfoliated graphite, especiallylaminates of sheets of compressed particles of exfoliated graphite, havebeen found particularly useful as heat spreaders for plasma displaypanels. In practice, this requires the graphite heat spreaders to beproduced with a layer of adhesive thereon to adhere the heat spreader tothe plasma display panel, especially during the plasma display panelassembly process. A release liner must then be used to overlay theadhesive, with the adhesive sandwiched between the release liner and thegraphite sheet, to permit storage and shipping of the graphite heatspreader prior to adhesion to the plasma display panel.

The use of an adhesive coated graphite sheet (or laminate of sheets)with a release liner has certain requirements which must be met if it isto be practical in a high volume plasma display panel manufacturingprocess. More particularly, the release liner must be capable of beingremoved from the sheet at high speed without causing delamination of thegraphite. Delamination occurs when the release liner in effect pulls theadhesive and some of the graphite off the sheet as it is being removed,resulting in a loss of graphite, impairment of the graphite sheetitself, and diminution of adhesive needed to adhere the graphite sheetto the plasma display panel, as well as an unsightly and unfortunateappearance.

With that however, though the adhesive and release liner must beselected to permit release of the release liner from theadhesive/graphite sheet without delamination of the graphite, theadhesive must still be strong enough to maintain the graphite sheet inposition on the plasma display panel while the panel assumes any of avariety of orientations and to ensure good thermal contact between theheat spreader(s) and the panel.

In addition, significant diminution of the thermal performance of theheat spreader must not be caused by the adhesive. In other words, anadhesive applied in a layer that is of substantial thickness caninterfere with the thermal performance of the heat spreader, since theadhesive would interfere with the conduction of heat from the plasmadisplay panel to the heat spreader.

Thus, the adhesive and release liner combination must achieve a balancesuch that they provide a release load no greater than about 40 g/cm,more preferably about 20 g/cm and most preferably about 10 g/cm, at arelease speed of about 1 m/s, as measured, for instance, on aChemInstruments HSR-1000 high speed release tester. For instance, if itis desired to remove the release liner at a speed of about 1 m/s inorder to match the high volume manufacturing requirements of the plasmadisplay panel, the average release load of the release liner should beno greater than about 40 g/cm, more advantageously, about 20 g/cm, andmost advantageously about 10 g/cm, in order to permit removal of therelease liner without causing graphite delamination at that releasespeed. To achieve this, the adhesive should most preferably be nogreater than about 0.3 mils in thickness.

Another factor to be balanced is the adhesion strength of the adhesivewhich as noted above, must be sufficient to maintain the heat spreaderin position on the plasma display panel during the plasma display panelmanufacturing process and to ensure good thermal contact between theheat spreader and the plasma display panel. In order to achieve therequired adhesion, the adhesive must have a minimum lap shear adhesionstrength of at least about 125 g/cm², more preferably an average lapshear adhesion strength of at least about 700 g/cm², as measured, forinstance, on a ChemInstruments TT-1000 tensile tester.

With all that, as noted above, the adhesive must not substantiallyinterfere with the thermal performance of the heat spreader. By this ismeant, the presence of the adhesive should not result in an increase inthe through-thickness thermal resistance of the heat spreader of morethan about 100% as compared to the heat spreader material itself,without adhesive. Indeed, in the more preferred embodiment, the adhesivewill not lead to an increase in the thermal resistance of more thanabout 35% as compared to the heat spreader material without adhesive.Thus, the adhesive must meet the release load requirements and averagelap shear adhesion strength requirement while being thin enough to avoidan undesirably high increase in thermal resistance. In order to do so,the adhesive should be no thicker than about 0.5 mils, more preferablyno thicker than about 0.25 mils.

In order to achieve the balancing described above needed for theproduction of a heat spreader useful for being applied to a plasmadisplay panel in a high volume manufacturing process, where the heatspreader is a sheet or laminate of sheets of compressed particles ofexfoliated graphite having a thickness no greater than about 2.0millimeters and a density between about 1.6 and about 1.9 grams percubic centimeter, and Aroset 3300 pressure sensitive acrylic adhesivecommercially available from Ashland Chemical in the desired thicknesscombined with a release liner made of silicone-coated Kraft paper suchas an L2 release liner commercially available from Sil Tech, a divisionof Technicote Inc., can achieve the desired results. Thus, a heatspreader composite is provided which comprises a heat spreader materialsuch as a shear or laminate of shears of compressed particles ofexfoliated graphite, having an adhesive thereon in a thickness such thatthe thermal performance of the heat spreader material is notsubstantially compromised, with a release layer positioned such that theadhesive is sandwiched between the heat spreader material and therelease material. In operation then, the release material can be removedfrom the heat spreader/adhesive combination and the heat spreadermaterial/adhesive combination then applied to a plasma display panelsuch that the adhesive adheres the heat spreader material to the plasmadisplay panel. Furthermore, when a plurality of plasma display panels isbeing produced, the at least one of the heat spreader/adhesivecombinations is applied to each of the plurality of plasma displaypanels.

In this manner, a superior heat spreader for plasma display panels isprovided in a manner such that high volume manufacturing of the plasmadisplay panels can be continued while the heat spreader is supplied andapplied to the panels.

In order to facilitate a more complete understanding of the invention, anumber of examples are provided below. However, the scope of theinvention is not limited to the specific embodiments disclosed in theseexamples which are for purposes of illustration only. All proportionsand quantities referred to in the following examples are by weightunless otherwise stated.

EXAMPLE 1

High speed release liner tests were performed using a ChemInstrumentsHSR-1000 high speed release tester. The test conditions were such thatthe drive wheel speed was set at 400, 800 feet/minute, a release angleof 180 degrees, surface release speed of 40, 80 inches/second, aspreader release rate of 0.5, 0.25 seconds and a sample size of 2 inchesby 8 inches.

The diagram of the high speed release test is shown in FIG. 1. Therelease liner 20 is peeled back slightly from one end exposing of thesample exposing the graphite heat spreader 10 and the release liner 20.The exposed end of the graphite 10 is firmly held in place with a clamp100 while a paper trailer 110 is attached to the free end of releaseliner 20. Paper trailer 110 is then double backed on itself and is fedbetween a drive wheel 120 and an idler wheel 125. The test involvesdriving the drive wheel 120 at a set speed and pressed against papertrailer 20, which is supported by idler wheel 125 illustrated in FIG. 2.Trailer 20, driven by driver wheel 120 moves at the same speed as drivewheel 120 pulling release liner 20 from graphite shear 10.

Because paper trailer 110 is double backed on itself, liner 20 isremoved from graphite shear 10 at a release angle of approximately 180degrees. This causes the adhesive interface to move along the surface ofgraphite sample 10 at one half the speed of paper trailer 110. As noted,tests were conducted at drive wheel 120 speeds of 400 and 800 feet perminute, corresponding to interface release speeds of 40 and 80 inchesper second. In turn, these speeds correspond to a release liner releasespeed of 0.5 and 0.25 seconds, respectively.

During each test, the maximum release force was recorded, in grams per 2inch width of sample. After testing, each sample was inspected for signsof graphite delamination, raised areas of graphite, or graphitetransfers to the release liner. A sample failed the test if any largeraised areas of graphite were observed or if there was any delaminationof the graphite. Results are set out in Table I for 2 different adhesivecompositions.

As shown in Table I, a total of 199 samples of eGraf 755 graphite heatspreader material, coated with Aroset 3250 adhesive were tested at arelease rate if 0.5 seconds. 163 passed the test while 36 failed, for apassing rate of 82%. The average of the maximum release force measuredon the samples that failed was 154 grams per 2 inch width, while theaverage of those that passed was 42 grams per 2 inch width.

A total of 12 samples coated with Aroset 3300 adhesive were tested, 8 ata release rate of 0.5 seconds and 4 at a release rate of 0.25 seconds.In both cases all samples passed. The average of the maximum releaseforce measured on those tested at the slower rate was only 17.4 gramsper 2 inch width while for the faster rate, the average was only 19.7grams per 2 inch width.

EXAMPLE 2

Lap shear adhesion tests were performed using ChemInstruments TT-1000tensile tester. Test conditions were set such that crosshead speed is0.5 inches/minute, lap shear area size is 1″ by 1″. eGraf specimen sizewas 1″ wide by 4″ long. Test substrate material glass and test substratesize 2″ wide by 4″ long. Specimens were die cut from each sample shearafter affixing the sample to the glass substrate, a 1,000 gram weightwas applied to the graphite/glass joint on the graphite side for 20minutes prior to testing. No other forces were applied to the joint.

Samples were assembled in the tensile tester with the glass substratelocated in the upper jaw and the sample located in the lower jaw. Testswere conducted at a crosshead speed of 0.5″ per minute. Maximum lapshear were obtained for each sample and are summarized in table II.

As shown in Table II, 100 samples of eGraf 755 graphite heat spreadercoated with Aroset 3250 adhesive were tested, along with 10 samples ofeGraf 755 heat spreader coated with Aroset 3300 adhesive. The average ofthe maximum lap shear strengths for the Aroset 3250 adhesive coatedsamples was 4129 grams while the average for the Aroset 3300 adhesivecoated samples was 3738 grams. The standard deviation for the Aroset3250 adhesive samples was 1422 grams or that of the Aroset 3300 adhesivesamples was 1822 grams. Thus, there is on average a 10 percent reductionin the lap shear strength when the Aroset 3300 adhesive is employed.

EXAMPLE 3

Probe tack tests were performed using a ChemInstruments PT-1000 ProbeTack Tester. Probe tack measures the initial “grab” of the adhesive on asubstrate under no load conditions. Probe tack loads were obtained forsamples of eGraf 755 graphite heat spreader coated with Aroset 3250adhesive and Aroset 3300 adhesive, respectively and the results aresummarized in Table III.

As shown in Table III, Probe Tack Tests were carried out on 26 samplescoated with Aroset 3250 adhesive and 16 samples coated with Aroset 3300adhesive. The Aroset 3250 samples came from 3 spreaders while the Aroset3300 samples came from 2 spreaders. The average of the Probe tack loadsfor the Aroset 3250 adhesive coated samples was 23 grams while theaverage for the Aroset 3300 adhesive coated samples was 19.1 grams. Thestandard deviation for the Aroset 3250 coated samples was 10.5 gramswhile that for the Aroset 3300 adhesive coated samples was 9.0 grams,indicating that on average, there is a 17% reduction in probe tack loadfor the 3300 adhesive compared to the Aroset 3250 adhesive.

EXAMPLE 4

Thru thickness thermal resistant tests were performed using the modifiedASTM D5470 Thermal Conductivity Test. The tests were carried out oneGraf graphite heat spreader without adhesive and eGraf 755 heatspreader with either Aroset 3250 adhesive or Aroset 3300 adhesiveapplied to one side only. Two samples of each material were tested. Thesamples were 2.0 inches in diameter and tests were conducted at acontact pressure of 16 psi and a nominal specimen temperature of 50° C.Test results are summarized in Table IV. As shown, eGraf 755 materialwithout adhesive had a thermal resistance of 3.48 cm²° C./W. The samplecoated with Aroset 3250 had a thermal resistance which varied between4.46 and 4.55 cm²° C./W. while that of the sample coated with Aroset3300 adhesive varied between 3.77 and 3.99 cm²° C./W, indicating thatthe thermal performance of the Aroset 3300 adhesive coated sample issubstantially better than that of the Aroset 3250 adhesive coatedsample.

The foregoing examples illustrate the balancing tests which need to beused to identify acceptable release liners and adhesives for use withheat spreader materials in order to accomplish the balanced heatspreader composite of the present invention.

All cited patents and publications referred to in this application areincorporated by reference.

The invention thus being described, it will obvious that it may bevaried in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the present invention and allsuch modifications as would be obvious to one skilled in the art areintended to be included in the scope of the following claims.

1. A method for applying heat spreaders to a plurality of plasma displaypanels, comprising (a) providing a plurality of heat spreadercomposites, each of which comprises a heat spreader material comprisingat least one sheet of compressed particles of exfoliated graphite havingan adhesive thereon and a release material positioned such that theadhesive is sandwiched between the heat spreader material and therelease material, wherein the adhesive and release material provide anaverage release load of no greater than about 40 grams per centimeter ata release speed of one meter per second; (b) removing the releasematerial from a plurality of the composites; and (c) applying at leastone of the composites to each of the plurality of plasma display panelseach such that the adhesive adheres the heat spreader material to theplasma display panel.
 2. The method of claim 1 wherein the releasematerial and adhesive are selected to permit a predetermined rate ofrelease of the release material without causing undesirable damage tothe heat spreader material.
 3. The method of claim 1 wherein the averagerelease load is no greater than about 10 grams per centimeter at arelease speed of one meter per second.
 4. The method of claim 1 whereinthe adhesive achieves a minimum lap shear adhesion strength of at leastabout 125 grams per square centimeter.
 5. The method or claim 4 whereinthe adhesive achieves an average lap shear adhesion strength of at leastabout 700 grams per square centimeter.
 6. The method of claim 4 whereinthe adhesive results in an increase in through thickness thermalresistance of the adhesive/heat spreader material of not more than about35% as compared to the heat spreader material itself.
 7. The method ofclaim 6 wherein the thickness of the adhesive is no greater than about0.5 mils.
 8. The method of claim 7 wherein the thickness of the adhesiveis no greater than about 0.25 mils.
 9. The method of claim 1 wherein theheat spreader material comprises a laminate comprising a plurality ofsheets of compressed particles of exfoliated graphite.
 10. A method forapplying a heat spreader to a heat source, comprising (a) providing aheat spreader composite, which comprises a heat spreader materialcomprising at least one sheet of compressed particles of exfoliatedgraphite having an adhesive thereon and a release material positionedsuch that the adhesive is sandwiched between the heat spreader materialand the release material, wherein the adhesive and release materialprovide an average release load of no greater than about 40 grams percentimeter at a release speed of one meter per second; (b) removing therelease material from the composite; and (c) applying the composite to aheat source such that the adhesive adheres the heat spreader material tothe heat source, wherein the release material and adhesive are selectedto permit a predetermined rate of release of the release materialwithout causing undesirable damage to the heat spreader material. 11.The method of claim 10 wherein the average release load is no greaterthan about 10 grams per centimeter at a release speed of one meter persecond.
 12. The method of claim 10 wherein the adhesive achieves aminimum lap shear adhesion strength of at least about 125 grams persquare centimeter.
 13. The method or claim 12 wherein the adhesiveachieves an average lap shear adhesion strength of at least about 700grams per square centimeter.
 14. The method of claim 12 wherein theadhesive results in an increase in through thickness thermal resistanceof the adhesive/heat spreader material of not more than about 35% ascompared to the heat spreader material itself.
 15. The method of claim14 wherein the thickness of the adhesive is no greater than about 0.5mils.
 16. The method of claim 15 wherein the thickness of the adhesiveis not greater than about 0.25 mils.
 17. The method of claim 10 whereinthe heat spreader material comprises a laminate comprising a pluralityof sheets of compressed particles of exfoliated graphite.